For each user in the user permissions list, you need to click on them to see their custom permissions. You can click Not Set (No) at the top of the column to set all permissions to that (ie the 'default') If you set any numerical permissions you will have to manually set these back to the default. Then save and the custom permissions should be gone.
If you are setting custom permissions for a lot of users, you'd be better setting up user groups with combinations of these permissions as required and adding users to these as needed. This is much easier to manage and easier to add and remove users.